This invention is related to methods and apparatus for measuring and controlling angles between objects and, more specifically, to a method and apparatus, including angle sensitive interferometer apparatus, for detecting and controlling angle changes in an incident beam of coherent radiation such as lasers, and utilizing same for measuring and controlling angles between objects and devices.
There are many circumstances or situations in which it is desireable to measure very accurately and even control very precisely the angles or relative positions and orientations between two or more objects or points. Laser beams, which are very intense, coherent beams of monochromatic light, have been used quite successfully for accurate angle detection and alignment application such as surveying devices and guidance systems. However, even these highly accurate laser instruments which are based on the nature of a laser beam to not spread out widely in space, are limited. There is a need for even more accurate angle measuring and control capability to detect and control more minute angle changes than possible with present laser and other devices for many applications, including command guidance systems for space vehicles, materials testing devices and the like. This invention utilizes radiation interference phenomenon to achieve such desired extremely accurage angle measuring and controlling results.
When two or more trains or beams of coherent electromagnetic radiation, such as laser light, microwaves, radio frequency waves, and the like, cross each other or are superimposed on each other, the resultant wave displacement at any point and at any instant is the sum of the instantaneous displacement that would be produced at the point by the individual wave trains if each was present alone. Where the respective waves are in phase with each other, their displacements would add resulting in increased intensity. However, where the respective waves are out of phase with each other, they cancel resulting in decreased intensity. Such increasing or decreasing of intensity by crossing or superimposing trains or beams of electromagnetic radiation on each other is referred to as interference. Where the waves are in phase and add respective displacements to increase intensity, the interference is constructive. where the waves are out of phase and their respective displacements cancel to decrease intensity, the interference is destructive.
It is known that when a beam of laser light or other coherent electromagnetic radiation is split into two parts to produce two beams and each of the two parts is made to travel a different path and then joined together again, they will produce a fringe pattern of intense, bright bands separated by less intense, dark bands. Such fringe patterns can be observed, and the distances between the bands in the fringe pattern are related to the wave lengths of the radiation. Thus, it has long been recognized that the wave lengths of light can be measured by producing an interference fringe pattern, measuring the separation of the bands in the fringe pattern, and, through mathematical equations, calculating the wave length of the light.
The essential feature of the formation of fringes is the division of a beam of light by partial reflection at a surface, and the subsequent recombination of the two disturbances or separated parts of the beam. Apparatus for producing interference fringes in this manner are referred to as interferometers.
There are a number of two-beam interferometers that have been developed and used for measuring the wavelength or frequency of radiation. For example, in the Michelson interferometer, which is well known to persons skilled in this art, the beam is divided into two beams of approximately equal intensity by a beam splitter. The beams are reflected at respective front silvered mirrors and recombined at the beam splitter. Other standard interferometer apparatus known to persons skilled in this art include the Twyman and Green interferometer, the Jamin interferometer, and the Mach-Zehnder interferometer. These standard known interferometers can be designed to have very high sensitivity for wavelength or frequency measurements. Other interferometer apparatus, such as the Koster's interferometer and Dowell's interferometer have been designed for such purposes as measuring slip gauges and distances very accurately. However, all of these prior art interferometer apparatus suffer from feedback and are severely limited in their angle sensitivity and effectiveness in angle measuring and controlling applications.
Feedback in prior art interferometers results from a portion of the radiation emitted from a radiation source, such as a laser, being reflected from the interferometer directly back into the laser source. Such beam or radiation feedback into the laser or radiation source produces interaction between the reflected light and the laser medium resulting in undesireable changes in frequency, intensity, and even the angle of the laser beam. Other detrimental effects in prior art devices can result from wavefront distortion due to physical limitations of size and structure of reflecting devices that limit the useable portion of beam diameter.
Another characteristic in the prior art interferometers, such as Michelson, Mach-Zehnder, and the like, is that the superimposed output beams are parallel even when the angle of the incoming incident beam is varied. Therefore, the intensity distribution in the fringe pattern of a normal Michelson interferometer or the like is described by the equation: EQU p.lambda.=2t cos .theta., (1)
where p is the order number of the interference fringe, .lambda. is the wavelength of the radiation (related to the radiation frequency .gamma. by the definition .lambda.=C/.gamma., where C is the speed of light), t is the difference in the optical path length for the two separated beams within the interferometer, and .theta. is the angle of incidence of the light beam on the interferometer. The change of intensity for a small angle .theta. is proportional to .theta..sup.2. Such a relationship does not provide a particularly angle-sensitive interferometer.